Pentacene thin-film transistors (TFTs) were fabricated on thermally grown SiO2 gate insulator under the conditions of various pre-cleaning treatments. Initial nucleation and growth of the material films on treated substrates were observed by atomic force microscope. The performance of fabricated TFT devices with different surface cleaning approaches was found to be highly related to the initial film morphologies. In contrast to the three-dimensional island-like growth mode on SiO2 under an organic cleaning process, a layer-by-layer initial growth occurred on the SiO2 insulator cleaned with ammonia solution, which was believed to be the origination of the excellent electrical properties of the TFT device. Field effect mobility of the TFT device could achieve as high as 1.0 cm(2)/Vs on the bared SiO2/Si substrate and the on/off ratio was over 10(6). (c) 2009 Elsevier B.V. All rights reserved.